This article will discuss repair and calibration of the NRI Professional model 70 tube tester. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.

NRI 70 beautiful hardwood case

Introduction

Built by Precision Apparatus Co for NRI, the high-quality craftsmanship of the NRI Professional model 70 tube tester is readily apparent. It has a beautiful hardwood case (appears to be oak or birch) with finger-joint construction. Eight rubber feet protect your desktop in both the working position and the standup position. Internally, the tester uses a large beefy transformer and quality Pace (Precision Apparatus) meter movement. The tester has very small size, measuring only 10.75 x 10.75 x 6.25 inches (27cm x 27cm x 16cm) and weighs a hefty 10.4 lbs (4.7 kg). Most of the weight is due to the large transformer and hardwood case.

Set of 3 PRECISION REFERENCE CAPACITORS – for capacitor tester rebuild (Sprague Tel-Ohmike, Heathkit, Eico, Knight, Paco, etc…) For those of you doing your own cap tester repair, I sell a set of precision reference capacitors for $33 free ship USA. You get the entire set of 3 reference caps (1 of 200 pf, 1 of 0.02 µf, and 1 of 2.00 µf). All caps accuracy 1% or better and the 200 pf cap will be variable for maximum flexibility.

You NEED precision reference capacitors to achieve an accurate capacitor tester. If you want a 2nd set of precision capacitors to rebuild another unit, or a set to test the accuracy of your own tester, you can buy two sets for $53 free ship USA.

Other values of precision capacitors are available by request. Email for details.

1 set of 3 precision caps – $33 free ship – YOU MUST SPECIFY YOUR MAKE-MODEL OF CAP TESTER IN THE (NOTES) SECTION WHEN MAKING PAYPAL PAYMENT

These units are known as a Resistance-Capacitance Bridge, an R/C tester, but are most commonly used as a capacitor tester. Extreme caution must be followed with any vintage capacitor tester because very high voltages are present during repair and while operating. They should only be used by knowledgeable technicians.

EICO 950A

PACO C-20 tester

Knight KG-670

Heathkit C-3 (dark color scheme)

Heathkit C-3 (light color scheme)

NRI Professional model 311

Eico 950B

These models use a balanced bridge that measures capacitance from 10mmf up to 5000mf and resistance from 0.5 Ω up to 500-MΩ. During component value testing, the magic eye tube serves as the null-indicator. When the bridge is far from balanced, the target area of the magic eye tube glows completely closed green, and in fact overlaps. As the pointer dial approaches balance point, first the overlapping disappears and eventually the entire target area of the eye tube is completely open (dark). The bridge is balanced when the maximum dark area is indicated, and you can then read the value of your component on the faceplate scale.

Most models use the magic eye tube for both Leakage testing and component value testing. With the Eico 950A, leakage of Paper/Mica caps is indicated by the #1629 magic eye tube, but electrolytic leakage is indicated by the Neon bulb.

This article discusses repair and calibration of the older tube-based Sencore Mighty Mite tube testers that use the 12AU7A tube inside, such as model TC130, TC136, and TC142. I will also discuss the most common problem that causes “faulty” Grid Leakage detection. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.

TC130 Mighty Mite case

Introduction

Sencore Mighty Mite testers employ a Cathode Emission test circuit, with short detection and industry-best 100-Megohm leakage detection. The leakage detection circuitry is really the reason that every technician should own a Mighty Mite as part of his/her tube testing arsenal.

The older Sencore Mighty Mite tube testers have tube circuitry inside, whereas newer Mighty Mites (such as TC154 and TC162) are transistorized. Otherwise, their functionality is comparable. Some units have a CRT picture tube wire harness with socket attached. This harness is very bulky, so common sense would suggest to remove it. It serves no practical purpose and only clutters up the case. Some models have a few pin straighteners on the front panel.

This article will discuss repair and calibration of the Jackson 561 tube tester. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.

Introduction

The Jackson 561 appears to be a rare (uncommon) model. Very little documentation exists, and I have seen only a small number of them for sale.

The model 561 is a combination of the Jackson 634 and the 648. Photo below shows Jackson 561 (right); Jackson 648A (left).

Comparison: Jackson 648A vs 561

If the 634 and 648 mated, the 561 would be its offspring. It shares the following characteristics from the 648: (a) the same meter, (b) variable leakage control, (c) color scheme, and (d) case design. From the 634, it shares (a) the same test method (which Jackson calls “Dynamic Output”), (b) Shorts Test control, (c) lack of a Noise test, and (d) function control layout. In fact, the 634 tube setup chart can be used (with slight modification) with the 561, as explained later.

To jump immediately to the section that explains the Upgrades and Mods that I find useful, click HERE.

BK 700

my BK 707 with custom TubeSound plate current meter upgrade

The B&K 707 tube tester dates from the late 1960s into the 1970s. My manual is stamped August 1969, and a modern tube data setup chart dates 1978.

First of all, a brief explanation of the circuitry of this tester is necessary. Is this a Mutual Conductance tube tester? Well, yes and no.

Tubes that test in the “Jet-Check” section test for mutual conductance, with the exception of diodes/rectifiers (which always test only for Emission). The Jet-Check section is the upper panel (sockets 1 – 35). Tubes that test in the “Switch” section — the bottom panel (sockets 36 – 45), are tested for Emission.

Discussion

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Safety

This website discusses repair of electronic items. Electrical voltages are present and represent a danger. Repairs should only be attempted by a qualified technician. We do not provide free personalized troubleshooting.